Printed circuit boards are generally formed of a rigid dielectric material which is printed with a predetermined pattern of an electrical conductor. Printed circuit boards may be electrically connected to one or more land grid array-type devices such as an application specific integrated circuit (ASIC) or a flexible printed circuit having an array of electrically conductive pads thereon. In order to electrically connect a land grid array-type device to a printed circuit board, an electrical connector or "socket" may be disposed therebetween which has an array of electrically conductive pads on each side thereof. The electrically conductive pads may be constructed from an elastomeric material. The pads on one side of the connector abut with the pads on the land grid array-type device, and the pads on the other side of the connector abut with the electrically conductive array on the printed circuit board.
In order to maintain electrical connection between a land grid array-type device and a printed circuit board, the device and the board must be compressed together, with the electrical connector therebetween. Such an assembly 10 is shown in FIG. 1. The surfaces 12, 14, respectively, of the device 20 (an ASIC being shown in this figure) and the board 22 that the electrical connector 24 is in between must each be flat to within a few mils of an inch. When pads 26, 28 (shown greatly enlarged for illustrative purposes) on an electrical connector 24 are compressed between a land grid array-type device 20 and a printed circuit board 22, these pads 26 (especially elastomeric ones) act as miniature springs, exerting forces "F0" opposing the compression of the device 20 and the board 22. Existing large-area connector arrays generate large forces between the printed circuit board and the device being attached to the board. These forces are often large enough to deflect the printed circuit board outside of the flatness requirements. Thus, in addition to needing a relatively large compressive force to maintain contact between the device, the connector and the board, a backing plate 30, FIG. 1, is required to support the printed circuit board 22 and maintain the flatness of the front surface 14 thereof. As shown in FIG. 1, such a backing plate 30 is usually positioned on the back side 16 of the printed circuit board 22, opposite the electrical connector 24 and land grid array-type device 20. A second backing plate 32, which may be part of a heat sink (not shown) or the like, may be positioned adjacent to the land grid array-type device 20.
As shown in FIG. 1, a biasing assembly 34 such as springs 36, 38 are generally required to maintain a large, relatively constant force "F1" on the board, connector and device. Such a biasing assembly 34 is usually placed on the top side 14 of the printed circuit board 22, adjacent to the second backing plate 32, as shown in FIG. 1. In general, with a linear spring, the force "F" provided by a spring is directly proportional to the spring constant "K" multiplied by the linear deflection "X" (F=KX). A spring having a low spring constant "K" is most desirable in this application in order to keep the spring force as consistent as possible. Specifically, manufacturing tolerances can vary among different installations. In addition, changes in environmental conditions such as temperature and creep of various components may cause the spring to deflect. Because of F=KX, a large spring constant "K" multiplied by even a small change in deflection "X" of the spring would produce a relatively large fluctuation in the force "F" provided by the spring.
Since a large force "F" is required and a low spring constant "K" is most desirable, the linear deflection "X" of any linear spring used in this application must be large. Furthermore, since a spring with more coils deflects a greater total distance than the same type of spring with fewer coils, a coil spring used in this application must be relatively long. Specifically with reference to FIG. 1, in order to provide a sufficient force "F1" to oppose the large forces "F0" generated by the pads 26 on the electrical connector 24, the length "L1" of each spring 36, 38 (shown compressed) must be relatively large. In today's small, densely-packed computers and electronics, the distance required for such springs 36, 38 may not be available on the top side 14 of a printed circuit board 22. Even if such a distance is available, providing a more compact biasing assembly is more desirable.
Thus, it is an object of the present invention to provide a backing plate assembly which includes a biasing assembly to provide a constant compressive force on a printed circuit board, electrical connector and land grid array-type device.
It is a further object of the present invention to provide a biasing assembly having a relatively low spring constant which provides a relatively large compressive force on a printed circuit board, electrical connector, and land grid array-type device, yet does not require a relatively large distance on the top or bottom side of the printed circuit board.
It is also an object of the present invention to provide a spring-loaded backing plate assembly as a single, compact unit positioned on the back side of a printed circuit board.
It is a further object of the present invention to use a simple, relatively low-cost leaf spring assembly, rather than a coil spring assembly, as the biasing assembly in a spring-loaded backing plate assembly.
It is a further object of the present invention to provide a spring-loaded backing plate assembly which provides a predetermined, constant force upon every installation thereof in a circuit assembly.